1. Field
Example embodiments relate to protection systems for and methods of operating nuclear boiling water reactor (“BWR”) power plants.
2. Description of Related Art
FIG. 1 illustrates a related art BWR. As shown, a pump 100 supplies water to a reactor vessel 102 housed within a containment vessel 104. The core 106 of the reactor vessel 102 includes a number of fuel bundles such as those described in detail below with respect to FIG. 2. The controlled nuclear fission taking place at the fuel bundles in the core 106 generates heat that turns the supplied water into steam. This steam is supplied from the reactor vessel 102 to turbines 108 that power a generator 110. The generator 110 then outputs electrical energy. The steam supplied to the turbines 108 is recycled by condensing the steam from turbines 108 back into water at a condenser 112, and supplying the condensed steam back to the pump 100.
FIG. 2 illustrates a typical fuel bundle 114 in the core 106. A core 106 may include, for example, anywhere from about 200 to about 900 of these fuel bundles 114. As shown in FIG. 2, the fuel bundle 114 may include an outer channel 116 surrounding a plurality of fuel rods 118 extending generally parallel to one another between upper and lower tie plates 120 and 122, respectively, and in a generally rectilinear matrix of fuel rods as illustrated in FIG. 3, which is a schematic representation of a cross-section or lattice of the fuel bundle 114 of FIG. 2. The fuel rods 118 may be maintained laterally spaced from one another by a plurality of spacers 124 vertically spaced apart from each other along the length of the fuel rods 118 within the outer channel 116. Referring to FIG. 3, there is illustrated in an array of fuel rods 118 (i.e., in this instance, a 10×10 array) surrounded by the outer channel 116. The fuel rods 118 are arranged in orthogonally related rows and also surround one or more “water rods,” two water rods 126 being illustrated. The fuel bundle 114 may be arranged, for example, in one quadrant of a control blade 128 (also known as a “control rod”). It will be appreciated that other fuel bundles 114 may be arranged in each of the other quadrants of the control blade 128. Movement of the control blade 128 up and/or down between the fuel bundles 114 controls the amount of reactivity occurring in the fuel bundles 114 associated with that control blade 128.
The total number of control blades 128 utilized varies with core size and geometry, and may be, for example, between about 50 and about 200. The axial position of the control blades 128 (i.e., fully inserted, fully withdrawn, or somewhere in between) is based on the need to control excess reactivity and to meet other operational constraints. For each control blade 128, there may be, for example, 24, 48, or more possible axial positions or “notches.”
The BWR may include several related art closed-loop control systems that control various individual operations of the BWR in response to demands. For example, a related art recirculation flow control system (“RFCS”) may be used to control core flowrate that, in turn, help to determine the output power of the reactor core. A control blade drive system affects the position of the control blades, the control blade density within the core, and core reactivity. A turbine control system controls steam flow from the BWR to the turbines based on load demands and pressure regulation.
The operation of all of these systems, as well as other related art systems, is controlled utilizing various monitoring parameters of the BWR. Exemplary monitoring parameters include core flow and flowrate effected by the RFCS, reactor vessel dome pressure (which is the pressure of the steam discharged from the pressure vessel to the turbines), neutron flux or core power, feedwater temperature and flowrate, steam flowrate provided to the turbines, and various status indications of the BWR systems. Many monitoring parameters are measured directly by related art sensors, while others, such as core thermal power, are typically calculated using measured parameters. These status monitoring parameters are provided as output signals from the respective systems.
Nuclear reactors are conservatively specified to minimize any risks from the hazardous materials involved in their use. The materials used in BWRs must withstand various loading, environmental, and radiation conditions. For example, operating pressures and temperatures for the reactor pressure vessel are about 7 MPa and 290° C. for a BWR. Reactor vessel walls are thus several inches thick and very strong materials are used for reactor components. Nonetheless, contingencies are required for failure as components are subjected to operational stress for decades. These contingencies involve not only many layers of preventive systems, but also procedures for rectifying problems that arise.
Related art reactor control systems have automatic and manual controls to maintain safe operating conditions as the demand is varied. The several control systems control operation of the reactor in response to given demand signals. Computer programs are used to analyze thermal and hydraulic characteristics of the reactor core. The analysis is based on nuclear data selected from analytical and empirical transients and accidents, and from reactor physics and thermal-hydraulic principles. In the event of an abnormal transient, the reactor operator usually is able to diagnose the situation and take corrective action based on applicable training, experience, and/or judgment. Whether the manual remedial action is sufficient depends upon the transient and upon the operator's knowledge and/or training. If the transient is significant (i.e., challenges any of the reactor safety limits), reactor trip (also referred to as reactor shutdown, scram, or full insertion of all control blades) may be required (the term “scram” is alleged to have originated in the early years of reactor development and operation as an acronym for “super-critical reactor ax-man”). Some transients may occur quickly (i.e., faster than the capability of a human operator to react). In such a transient, a reactor trip will be initiated automatically. Safety analyses generally show that no operator action is necessary within 10 minutes of a postulated transient.
A related art nuclear reactor protection system (“RPS”) comprises a multi-channel electrical alarm and actuating system that monitors operation of the reactor and, upon sensing an abnormal transient, initiates action to prevent an unsafe or potentially unsafe condition. At minimum, the related art RPS typically provides three functions: (1) reactor trip that shuts down the reactor when certain monitored parameter limits are exceeded; (2) nuclear system isolation that isolates the reactor vessel and all connections penetrating a containment barrier; and (3) engineered safety feature actuation that actuates related art emergency systems such as cooling systems and residual heat removal systems.
Core power protection schemes are typically employed in BWRs when the reactor is operating in its normal operating domain (i.e., after startup and heatup of the reactor). FIG. 4 is a typical BWR power-to-flow operating map showing an operating domain of the reactor. Such operating domains are discussed, for example, in U.S. Pat. No. 5,528,639 (“the '639 patent”). After startup and heatup, the permissible operating domain for the BWR typically is above the cavitation region, below the maximum operating line, and bounded by the minimum normal flow line and the maximum normal flow line. In related art RPSs, when the BWR is operating within the operating domain, an unplanned transient that does not increase the power level (i.e., neutron flux) above a setpoint or setpoints associated with the maximum operating line will not cause a reactor overpower protection trip. FIG. 5 is a BWR power-to-flow operating map showing an operating domain of the reactor with expanded limits. Such operating domains are discussed, for example, in U.S. Pat. Nos. 6,721,383 B2 (“the '383 patent”) and 6,987,826 B2 (“the '826 patent”). FIG. 6 is a BWR power-to-flow operating map showing another operating domain of the reactor with expanded limits. Such operating domains are also discussed, for example, in the '383 patent and the '826 patent. The disclosures of the '639 patent, the '383 patent, and the '826 patent are incorporate in this application by reference.
A reactor overpower protection trip is initiated for certain transients that could cause an increase in power above the maximum safe operating level. Generally, an overpower equal to about 120% of the rated power can be tolerated without causing damage to the fuel rods. If thermal power should exceed this limiting value (the maximum safe operating level) or if other abnormal conditions should arise to endanger the system, the RPS will cause a reactor trip.
An essential requirement of an RPS is that it must not fail when needed. Therefore, unless the operator promptly and properly identifies the cause of an abnormal transient in the operation of the reactor, and promptly effects remedial or mitigating action, related art RPS will automatically effect reactor trip. However, it is also essential that reactor trip be avoided when it is not desired or necessary (i.e., when there is an error in the instrumentation or when the malfunction is small enough that reactor trip is unnecessary).
As discussed in U.S. Pat. No. 5,528,639 (“the '639 patent”), for example, four power-related methods may be used to ensure that acceptable fuel and reactor protection are maintained. Each method uses monitored neutron flux to sense when an increase in power occurs, but each employs a different approach to initiate reactor trip.
The first method of protection causes a reactor overpower protection trip if the monitored neutron flux exceeds a preselected and fixed first setpoint. This first setpoint may be, for example, about 120%-125% of rated power.
The second method of protection causes a reactor overpower protection trip if the monitored neutron flux exceeds a preselected, but flow-referenced, second setpoint. In this method, the second setpoint is equal to the first setpoint when the reactor core flow is high. However, when reactor core flow is reduced, the second setpoint is also reduced.
The third method of protection involves electronically filtering the measured neutron flux signal to produce a signal that has been called simulated thermal power (“STP”). Usual practice employs a single-time-constant filter that approximates the thermal response of the reactor fuel rods. A reactor overpower protection trip is initiated when the STP signal exceeds the flow-referenced second setpoint. This second setpoint may be, for example, about 110%-115% of rated power. The third method is usually used in combination with the first method.
In the three methods discussed above, the reactor overpower protection trip setpoints are above the normal operating domain of the reactor to avoid undesired trips during operation in the upper portion of the operating domain. If more protection is required due to partial core power and flow conditions, the reactor overpower protection trip setpoints are manually adjusted. These manual adjustments are a cumbersome nuisance for reactor operators. However, if the reactor overpower protection trip setpoints are not adjusted, complex and restrictive core operating limits are required to ensure acceptable protection at all operating power and flow conditions.
Slow transients have been postulated in the partial power and flow range that challenge the effectiveness of these three related art protection methods. These slow transients have been postulated to avoid the protection provided by the associated reactor overpower protection trip setpoints.
As discussed in the '639 patent, a fourth method of protection involves automatically adjusting reactor overpower protection trip setpoints to be a controlled margin above the operating power level of the BWR. The fourth method provides enhanced reactor protection when the reactor is operating at less than the maximum operating level. However, alternate and/or supplemental methods of protection may be desired.